To investigate delay time behavior of a digital circuit, various devices are used, apart from expensive test devices located outside of the digital circuit. One device usually arranged within the digital circuit is based on a ring oscillator of inverting cells which are constructed using the same technology as the digital circuit to be evaluated. A rough assessment of the delay time behavior can be given by way of measurements at the ring oscillator. However, this circuit provides only a rough assessment of the delay time behavior, and thus does not directly measure the delay time behavior of a section of the digital circuit that is to be determined but can only indirectly make a statement about the delay time behavior of the overall digital circuit.
In the other technique, an analog-digital converter (ADC) is used to examine a wave form of a critical signal of the digital circuit. While the delay time behavior of a section of the digital circuit can be directly investigated, such a technique is very expensive as well as taking up a large portion of valuable real estate of the digital circuit.